This invention relates generally to gas turbine engines, and more particularly, to thrust directing apparatus for aircraft engines applied to aircraft of the VTOL (vertical takeoff and landing) type.
The so-called VTOL aircraft embodies a concept which incorporates the advantages of both the fixed wing aircraft and the helicopter to enable both vertical takeoff and landing and efficient horizontal high-speed flight. This flexibility is generally obtained by the use of gas turbine engines to develop thrust which is, in turn, directed to either the vertically downward (lift) or horizontally rearward (cruise) directions. Since it is inefficient to provide separate engines for lift and cruise, a common approach is to use a plurality of gas turbine engines which are adaptable to varying their thrust forces from the vertical to the horizontal direction by way of appropriate thrust deflecting means such as, for example, a clam-shell nozzle, or by way of separate lift augmenter devices such as, for example, remote gas-coupled turbotip fans, gas-interconnected ejectors or shaft-interconnected fans.
During the vertical takeoff or landing mode of operation, the propulsion system must provide not only the total lift force substantially equal to the aircraft gross landing weight but also must provide the proper force distribution in order to maintain adequate aircraft attitude control. Traditionally, this stability has been provided by the use of at least three lift-producing mechanisms or fans, one in each wing and one in the nose of the aircraft. The three remote lift/cruise fans are driven by a pair of gas turbine engines and are operated in the lift mode only during takeoff and landing. For horizontal flight, the thrust may be directed rearwardly directly from the gas turbine engines, or the fans located in the wings may be modulated to direct the thrust rearwardly. But the remote fan located in the nose section of the aircraft can only be used for lift and contributes no propulsive thrust during the period of horizontal flight operation.
In addition to the inefficiency of having a fan which is only used during the lift mode of operation, another reason why it is undesirable to use remote fans is that the size that is required for the large thrust levels necessarily has a limited tip speed and thus the shaft speeds are accordingly limited. This, in turn, makes it difficult to obtain an efficient match-up between the fan and the engine turbine design, unless reduction gears are employed. Reduction gears are undesirable because of increased weight and system complexity.
Another problem associated with VTOL-type aircraft is the difficulty in transitioning between the vertical and horizontal modes of operation. That is, when transitioning from the horizontal to the vertical mode of operation, it is necessary to simultaneously reduce the horizontal thrust from a high level to essentially zero and increase the vertical thrust from zero to a high level. At the same time, it is necessary to maintain the attitude of the aircraft in a stable position. This, of course, must be accomplished with a very high margin of safety.
It is therefore an object of the present invention to provide a gas turbine engine which can be used to produce thrust for both lift and cruise conditions of a VTOL aircraft.
Another object of the present invention is the provision in a VTOL aircraft for eliminating the need of remote fans used for lift purposes only.
Yet another object of the present invention is the provision in VTOL aircraft for the elimination of a remote fan in the nose section of the aircraft.
Still another object of the present invention is the provision in a VTOL aircraft for providing pitch control with the use of a single engine.
Yet another object of the present invention is the provision of a turbofan engine which can selectively provide either horizontal or vertical thrust.
Still another object of the present invention is the provision in a turbofan for effectively and safely transitioning between horizontal and vertical thrust modes of operation.
These objects and other features and advantages become more readily apparent upon reference to the following description when taken in conjunction with the appended drawing.